The present invention relates to methods and apparatus utilizing a roll type, twin wire former for producing a web. More particularly, it relates to methods and apparatus of this character which provide for simple and highly effective two-sided drainage of a jet of stock injected between two endless tensioned wires or foraminous belts adapted to run over a circumferentially grooved, rotatable forming cylinder.
For the sake of simplicity, the terms "wires" and "foraminous belts" will be used interchangeably herein. Thus, the word "wire" is intended to include not only conventional paper-making wires but also other kinds of foraminous belts used in web formers.
Roll type, twin wire formers have been used heretofore in which two-sided drainage has been achieved by employing forming cylinders designed as suction rolls. At high machine speeds, no water enters the interior of the forming cylinder, due to the resultant large centrifugal force, but it collects inside the shell and is expelled tangentially at the point where the wires between which the web is formed leave the forming cylinder. In such apparatus, the drainage is substantially more effective outwardly than inwardly of the forming cylinder. To prevent the paper web thus produced from exhibiting a pronounced two-sideness, it has been considered necessary to maintain inside the forming cylinder a vacuum substantially balancing the centrifugal force even at high machine speeds. However, the cost of such a forming cylinder does not favor its use and it has the disadvantage of producing a high sound level.
it has been proposed, therefore U.S. Pat. No. 4,028,175 (German Pat. No. 2,501,534), to use in place of a suction roll type of forming cylinder, a circumferentially grooved forming cylinder, and to balance the centrifugal force by providing a compressed air chest in the drainage zone, radially outside the forming cylinder and open towards it, to produce a counter-pressure increasing in steps in the direction of the forming cylinder. In this manner, it is said that the effect of the centrifugal force can be counterbalanced and even overcome by external compressed air. The intention is to obtain drainage that is symmetrical, of equal magnitude outwardly and inwardly, at every point in the drainage zone along the part of the forming cylinder over which the forming wires run. The bottoms of the circumferential grooves may be provided with holes to enable the air displaced by the water to escape into the cylinder. Also, the portion of the forming cylinder not in engagement with the running wires and the entire interior of the cylinder can be subjected to a vacuum from a save-all pan positioned at that portion of the forming cylinder. Such apparatus is also relatively expensive both in cost and in operation.
It has also been suggested that the web forming wires in the apparatus just described be led from the run-off point on the forming cylinder to a roll positioned under the cylinder, from which the inner wire relative to the forming cylinder (the outer wire relative to the roll) leaves at an early stage. This results in a detrimental partial adhesive of the wet web on the wrong wire and damage to the web from water expelled from the forming cylinder grooves and running through the inner wire. A doctor can be provided to eliminate web damage due to water but experience shows that it cannot be made efficient enough to prevent such damage.